They got mad at her when an item was missing out of a 4-bag $80 order (they unbagged and checked everything there on the counter).

That one seems valid. That person got burned before with the staff not bothering to do their job and were NOT going to short their friend whatever item(s) the staff kept for themselves. Sure, you can say the counter girl didn't do the bagging, but she's the one that the customer is supposed to tell, and it is hard not to be angry when you've paid for stuff and you're getting shorted -- AND there's almost surely another person relying on you to get it right this time. It shouldn't take so much effort to just get the stuff you paid for.